Demand for hydrogen is expected to increase rapidly in many industrial fields. Under these circumstances, hydrogen-permeable films are being developed that can be used to refine hydrogen gas. Known hydrogen-permeable films include hydrogen-permeable films using palladium (Pd) film. Pd is a rare precious metal and extremely expensive.
Therefore, less expensive materials are being developed as substitutes for Pd. For example, a hydrogen-permeable membrane unit has been proposed which comprises a hydrogen-permeable film consisting of niobium (Nb), vanadium (V), tantalum (Ta) and the like formed on the surface of a porous support that is permeable to gas (Patent Document 1). Patent Document 2 proposes a hydrogen permeating membrane using an alloy of zirconium (Zr) with nickel, chromium, iron, copper, vanadium, titanium or the like.
However, all substitutes for Pd are inferior to Pd in terms of hydrogen permeability. Moreover, as the hydrogen permeates the Pd substitutes they react with the hydrogen, causing hydrogenation and pulverization of the substitutes. Consequently, these substitutes are less durable than Pd.
Hydrogen-permeable films using Pd and Pd substitutes as hydrogen-permeable materials have film thicknesses on the order of a few microns to a few millimeters. In the past, only such extremely thick films have been developed as hydrogen-permeable films. In particularly, hydrogen-permeable films using only Pd as the film material are mechanically weak, and therefore cannot be made thinner than this. Moreover, because hydrogen-permeable materials (particularly Pd) are extremely expensive, manufacturing costs are high if the hydrogen-permeable film has to be thick.
The inventors in this case have done exhaustive research in order to obtain a hydrogen-permeable film that can provide three improved features in terms of functionality, durability and manufacturing costs. As a result, they arrived at a hydrogen-permeable film consisting of Pd or other hydrogen-permeable metal particles dispersed in a ceramic material consisting of a nitride or oxide of aluminum (Al) or silicon (Si), and submitted a previous application (Patent Document 3).
In this hydrogen-permeable film, Pd or other hydrogen-permeable metal particles are roughly uniformly dispersed in a hard ceramic material.
When hydrogen passes through this hydrogen-permeable film, the volume of the hydrogen-permeable metal particles alters as they absorb and then release hydrogen. However, because the hydrogen-permeable metal particles are dispersed in a hard ceramic material, the mechanical stress generated by this change in volume is absorbed and mitigated by the ceramic material. As a result, the hydrogen-permeable film is more durable because it undergoes less deterioration due to changes in the volume of the hydrogen-permeable metal. In addition, the hydrogen-permeable film is cheaper to manufacture because less of the hydrogen-permeable metal is used.
Patent Document 1: Japanese Patent Application Laid-open No. 2002-336664 (Claims)
Patent Document 2: Japanese Patent Application Laid-open No. 1995-000775 (Claims)
Patent Document 3: Japanese Patent Application Laid-open No. 2005-270966 (Claims)